The present invention relates to hydraulic drive systems, and in particular to a hydraulic drive system for a construction machine, such as a hydraulic excavator and hydraulic crane, having a plurality of working elements, which enables a variety of combined operation of there working elements to be performed with less number of hydraulic pumps.
The hydraulic drive system of such type is known from JP-A-58-146632, for example, which corresponds to U.S. Pat. No. 4,561,824 and EP-A-059471. This known hydraulic drive system comprises two hydraulic circuits each having a valve group including a hydraulic pump, travel directional control valve, swing directional control valve, boom directional control valve, arm directional control valve, and bucket directional control valve, which are connected to the respective hydraulic actuators such as travel motor, swing motor, boom cylinder, arm cylinder and bucket cylinder. Such connection of a plurality of valves to each of the boom cylinder, arm cylinder, bucket cylinder, etc., enables simultaneous driving of the travel motors and the other actuators to be performed substantially independently of each other for combined operation of the travel devices and the other working elements and also simultaneous driving of those other actuators to be performed substantially independently of each other for combined operation of the other working elements, as well as a single driving of each of the other actuators to be performed with two pumps for high speed operation of the associated working element.
However, the hydraulic drive system with such structure has a drawback that the manufacture cost is relatively expensive since the number of directional control valves connected to the actuators such as boom cylinder, arm cylinder, bucket cylinder, etc., must be increased